The present invention relates to polyolefin pipes made from a multimodal resin, a high density ethylene copolymer resin per se, and a process for preparing the resin.
There are a number of recent developments relating to polymers made using catalysts having a single or uniform active site such as a metallocene catalyst or constrained geometry catalyst. One class of polymers made using these types of catalysts systems is polyethylene, typically low density polyethylene. In some cases the polymers may contain long chain branching. The present invention relates to high density copolymers of ethylene.
Representative of the above art are Exxon""s United States Patents U.S. Pat. Nos. 5,382,630 and 5,832,631 both issued Jan. 17, 1995 to Stehling et al. The patents disclose blends of linear low density polyethylene. The blend per se has a polydispersity (Mw/Mn) greater than 3 while each component in the blend has a polydispersity of less than or equal to 3 and a different average molecular weight. The 631 patent is restricted to blends having a density from 0.88 to 0.900 g/cm3, which is well below the density of the blends of the present invention. The 630 patent claims blends having a density greater than 0.900 g/cm3, preferably from 0.900 to 0.940 g/cm3, which is essentially free of components having a higher average molecular weight and lower average comonomer content than that of any other component in the resin. This later feature is essentially directed to xe2x80x9creverse comonomer incorporationxe2x80x9d. Typically with conventional catalysts at a higher molecular weight there is a reduced tendency for the incorporation of the comonomer. (Or put the other way, the comonomer incorporation tends to be higher in the low molecular weight component of the polymers.) Additionally, the references disclose polymerizations at relatively low temperatures, such as gas phase reactions at temperatures from 50xc2x0 C. to 120xc2x0 C. and slurry polymerizations at temperatures up to 110xc2x0 C. The references do not disclose polymerizations at temperatures greater than 120xc2x0 C.
Applicants have been unable to locate any art disclosing the use of multimodal resins in the manufacture of pipe.
The EXXON patents disclose producing the blends xe2x80x9cin situxe2x80x9d by reaction with a metallocene catalyst. Interestingly the patents teach the polymer may be made using a gas phase, high pressure, slurry or solution polymerization. However, the reference fails to teach a dual reactor process in which the reactors are operated at different temperatures, nor does the reference suggest polymerization temperatures greater than 120xc2x0 C.
The present invention provides a multimodal polyolefin resin comprising not less than 80 weight % of ethylene and up to 20 weight % of one or more C3-10 alpha olefins. The said resin has a density greater than 0.940 g/cm3; a melt index determined according to ASTM D 1238 under a load of 2.16 kg; a temperature of 190xc2x0 C. greater than 0.01 g per 10 minutes; a polydispersity greater than 3.5; and a CDBI of greater than 50 comprising at least:
a) from 5 to 50 weight % of a high molecular weight component having a density greater than 0.940 g/cm3; a weight average molecular weight greater than 250,000; a polydispersity from 1.5 to 3.5; and a short chain branch content from 0.0 to 4.4 short chains per 1000 carbon atoms in the polymer backbone; and
b) from 50 to 95 weight % of a lower molecular weight component having a density from 0.930 to 0.960 g/cm3; a weight average molecular weight greater than 25,000; a polydispersity from 2.0 to 3.5; and a short chain branch content from 0.5 to 6.1 short chains per 1000 carbon atoms in the polymer backbone.
The present invention further provides a polyolefin pipe having a hydrostatic design basis at 23xc2x0 C. of equal or greater than 1250 psi and a ductile-brittle failure transition of at least 2000 hours when measured at 80xc2x0 C. under a minimum hoop stress of 900 psi prepared from the above composite resin.
The present invention further provides a process for preparing the above composite resin using a solution polymerization process in a series or parallel of two or more reactors at different temperatures each of which is not greater than 250xc2x0 C.